THE LOWER VERTEBRATA. 167 



Additional points to be noticed are : 



(1) The otic capsule (= periotic bone) of the dog ossifies 

 from three centres, one of which is equivalent to the 

 frog's pro-otic. 



(2) The several constituents of the lower jaw are not to 

 be distinguished in the adult mammal, which consists of a 

 single membrane-bone (on either side), Meckel's cartilage 

 altogether disappearing during the course of development. 



(3) The frog has no lachrymal or tympanic bones. 



(4) The zygomatic arch of the dog is not represented in 

 the frog. 



9. Metamerism of Skull. We are now in a position 

 to notice, without any danger of misconception, what is 

 called the segmental theory of the skull. Older anatomists, 

 working from adult structure only, conceived the idea that 

 the brain-case of the mammal represented three inflated 

 vertebroe. The most anterior had the pre-sphenoid for its 

 centrum, the orbito-sphenoids for the sides of its neural 

 arch, which was completed above by the f rentals (frontal 

 segment). Similarly, the basi-sphenoids, ali-sphenoids, and 

 parietals formed a second vertebra (parietal segment), and 

 the basi-, ex-, and supra-occipitals a third (occipital segment). 

 If this were correct, in the frog, which is a more primitive 

 rendering of the vertebrate plan, we should find the verte- 

 bral characters more distinct. But, as a matter of fact, as 

 the student will perceive, frontal segment, parietal segment, 

 and occipital segment can no longer be traced ; and the 

 mode of origin from trabeculse and parachordals shows very 

 clearly the falsity of this view. The vertebrate cranium is 

 entirely different in nature from vertebrae. The origin of 

 the parietals and frontals as paired bones in membrane 

 reinforces this conclusion. 



But as certainly as we have no such metameric seg- 

 mentation as this older view implies, in the brain-case of 

 the frog, so quite as certainly is metameric segmentation 

 evident in its branchial arches. We have the four gill-slits 

 of the tadpole and their bars repeating one another ; the 

 hyoid bar in front of these is evidently of a similar nature ; 

 and the view that the ear-drum is derived from an imperf orate 



